U.S. patent application number 16/756385 was filed with the patent office on 2020-09-03 for monophasic stimulation pulses with alternating polarity and extraordinary polarity changes.
The applicant listed for this patent is MED-EL Elektromedizinische Geraete GmbH. Invention is credited to Ernst Kabot.
Application Number | 20200276443 16/756385 |
Document ID | / |
Family ID | 1000004897249 |
Filed Date | 2020-09-03 |
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United States Patent
Application |
20200276443 |
Kind Code |
A1 |
Kabot; Ernst |
September 3, 2020 |
Monophasic Stimulation Pulses with Alternating Polarity and
Extraordinary Polarity Changes
Abstract
Arrangements are described for generating electrode stimulation
signals to electrode contacts in an implanted cochlear implant
electrode array. Electrode stimulation signals are a sequence of
monophasic stimulation pulses varying in polarity between positive
polarity and negative polarity with successive pulses separated in
time by an interpulse interval sufficient for neural response.
Accumulated charge imbalance and charge imbalance polarity are
calculated for each electrode contact after each stimulation pulse.
For each electrode contact a stimulation pulse has the same
polarity as an immediately preceding stimulation pulse for that
electrode contact only when the charge imbalance polarity has
opposite polarity from the immediately preceding stimulation pulse
for that electrode contact, and the accumulated charge imbalance
exceeds a defined charge imbalance threshold value. Otherwise, each
stimulation pulse has the opposite polarity as the immediately
preceding stimulation pulse for that electrode contact.
Inventors: |
Kabot; Ernst; (Innsbruck,
AT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MED-EL Elektromedizinische Geraete GmbH |
Innsbruck |
|
AT |
|
|
Family ID: |
1000004897249 |
Appl. No.: |
16/756385 |
Filed: |
November 12, 2018 |
PCT Filed: |
November 12, 2018 |
PCT NO: |
PCT/US2018/060258 |
371 Date: |
April 15, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62585104 |
Nov 13, 2017 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61N 1/0476 20130101;
G10L 25/48 20130101; A61N 1/36175 20130101; A61N 1/36038 20170801;
A61N 1/0541 20130101 |
International
Class: |
A61N 1/36 20060101
A61N001/36; A61N 1/05 20060101 A61N001/05; A61N 1/04 20060101
A61N001/04 |
Claims
1. A signal processing system for a hearing implant system having
an implanted electrode array with a plurality of stimulation
contacts for delivering electrode stimulation signals to adjacent
auditory neural tissue, the system comprising: a band pass filter
bank configured for processing an audio input signal to generate a
plurality of band pass signals each representing an associated band
of audio frequencies in the audio input signal; a stimulation
signal processor configured for generating electrode stimulation
signals for the electrode contacts based on the band pass signals,
wherein for each electrode contact, the electrode stimulation
signal is a sequence of monophasic stimulation pulses varying in
polarity between positive polarity and negative polarity with
successive pulses separated in time by an interpulse interval
sufficient for neural response; and a charge imbalance module
configured for calculating accumulated charge imbalance and charge
imbalance polarity for each electrode contact after each
stimulation pulse; wherein for each electrode contact: i. a
stimulation pulse has the same polarity as an immediately preceding
stimulation pulse for that electrode contact only when: (1) the
charge imbalance polarity has opposite polarity from the
immediately preceding stimulation pulse for that electrode contact,
and (2) the accumulated charge imbalance exceeds a defined charge
imbalance threshold value, and ii. otherwise, each stimulation
pulse has the opposite polarity as the immediately preceding
stimulation pulse for that electrode contact.
2. The signal processing system according to claim 1, wherein the
charge imbalance module is configured for calculating the
accumulated charge imbalance in terms of maximum comfortable level
(MCL) for each electrode contact.
3. The signal processing system according to claim 1, wherein the
defined charge imbalance threshold value is defined in terms of
maximum comfortable level (MCL) for each electrode contact.
4. The signal processing system according to claim 1, wherein each
sequence of monophasic stimulation pulses ends with a final charge
balancing stimulation pulse having a polarity and amplitude
offsetting the accumulated charge imbalance and charge imbalance
polarity so that after the final charge balancing stimulation pulse
the accumulated charge imbalance is zero.
5. The signal processing system according to claim 1, wherein the
stimulation pulses have a constant pulse width.
6. The signal processing system according to claim 1, where the
stimulation pulses have a variable pulse width.
7. The signal processing system according to claim 1, wherein the
interpulse interval is a fixed time duration.
8. The signal processing system according to claim 1, wherein the
interpulse interval is a variable time duration.
9. The signal processing system according to claim 1, wherein the
charge imbalance module is located in an implanted stimulation
processor implanted under the skin of a patient user.
10. The signal processing system according to claim 1, wherein the
charge imbalance module is located in an external signal processor
attached to the skin of a patient user.
11. A computer based method implemented using at least one hardware
implemented processor for generating electrode stimulation signals
to electrode contacts in an implanted cochlear implant electrode
array, the method comprising: using the at least one hardware
implemented processor to perform the steps of: processing an audio
input signal to generate a plurality of band pass signals, each
band pass signal representing an associated range of audio
frequencies; generating electrode stimulation signals for the
electrode contacts based on the band pass signals, wherein for each
electrode contact, the electrode stimulation signal is a sequence
of monophasic stimulation pulses varying in polarity between
positive polarity and negative polarity with successive pulses
separated in time by an interpulse interval sufficient for neural
response; and calculating accumulated charge imbalance and charge
imbalance polarity for each electrode contact after each
stimulation pulse; wherein for each electrode contact: i. a
stimulation pulse has the same polarity as an immediately preceding
stimulation pulse for that electrode contact only when: (1) the
charge imbalance polarity has opposite polarity from the
immediately preceding stimulation pulse for that electrode contact,
and (2) the accumulated charge imbalance exceeds a defined charge
imbalance threshold value, and ii. otherwise, each stimulation
pulse has the opposite polarity as the immediately preceding
stimulation pulse for that electrode contact.
12. The method according to claim 11, wherein the accumulated
charge imbalance is calculated in terms of maximum comfortable
level (MCL) for each electrode contact.
13. The method according to claim 11, wherein the defined charge
imbalance threshold value is defined in terms of maximum
comfortable level (MCL) for each electrode contact.
14. The method according to claim 11, wherein each sequence of
monophasic stimulation pulses ends with a final charge balancing
stimulation pulse having a polarity and amplitude offsetting the
accumulated charge imbalance and charge imbalance polarity so that
after the final charge balancing stimulation pulse the accumulated
charge imbalance is zero.
15. The method according to claim 11, wherein the stimulation
pulses have a constant pulse width.
16. The method according to claim 11, where the stimulation pulses
have a variable pulse width.
17. The method according to claim 11, wherein the interpulse
interval is a fixed time duration.
18. The method according to claim 11, wherein the interpulse
interval is a variable time duration.
19. The method according to claim 11, wherein the accumulated
charge imbalance and charge imbalance polarity are calculated by an
implanted stimulation processor implanted under the skin of a
patient user.
20. The method according to claim 11, wherein the accumulated
charge imbalance and charge imbalance polarity are calculated by an
external signal processor attached to the skin of a patient user.
Description
[0001] This application claims priority from U.S. Provisional
Patent Application 62/585,104, filed Nov. 13, 2017, which is
incorporated herein by reference in its entirety.
TECHNICAL FIELD
[0002] The present invention relates to signal processing
arrangements for hearing implants, and more particularly, to speech
coding strategies for cochlear implants.
BACKGROUND ART
[0003] As shown in FIG. 1, sounds are transmitted by a human ear
from the outer ear 101 to the tympanic membrane (eardrum) 102,
which moves the bones of the middle ear 103 (malleus, incus, and
stapes) that vibrate the oval window and round window openings of
the cochlea 104. The cochlea 104 is a long fluid-filled duct wound
spirally about its axis for approximately two and a half turns. It
includes an upper channel known as the scala vestibuli and a lower
channel known as the scala tympani, which are connected by the
cochlear duct. The cochlea 104 forms an upright spiraling cone with
a center called the modiolus where the spiral ganglion cells of the
acoustic nerve 113 reside. In response to received sounds
transmitted by the middle ear 103, the cochlea 104 functions as a
transducer to generate electric pulses which are transmitted to the
cochlear nerve 113, and ultimately to the brain which perceives the
neural signals as sound.
[0004] Hearing is impaired when there are problems in the ability
to transduce external sounds into meaningful action potentials
along the neural substrate of the cochlea 104. To improve impaired
hearing, hearing prostheses have been developed. In some cases,
hearing impairment can be addressed by a cochlear implant (CI), a
brainstem-, midbrain- or cortical implant that electrically
stimulates auditory neural tissue with small currents delivered by
multiple electrode contacts distributed along an implant electrode.
For cochlear implants, the electrode array is inserted into the
cochlea 104. For brain-stem, midbrain and cortical implants, the
electrode array is located in the auditory brainstem, midbrain or
cortex, respectively.
[0005] FIG. 1 shows some components of a typical cochlear implant
system where an external microphone provides an audio signal input
to an external signal processor 111 which implements one of various
known signal processing schemes. For example, signal processing
approaches that are well-known in the field of cochlear implants
include continuous interleaved sampling (CIS) digital signal
processing, channel specific sampling sequences (CSSS) digital
signal processing, spectral peak (SPEAK) digital signal processing,
fine structure processing (FSP) and compressed analog (CA) signal
processing. The processed signal is converted by the external
signal processor 111 into a digital data format, such as a sequence
of data frames, for transmission by an external coil 107 into a
receiving stimulation processor 108. Besides extracting the audio
information, the receiver processor in the stimulation processor
108 may perform additional signal processing such as error
correction, pulse formation, etc., and produces a stimulation
pattern (based on the extracted audio information) that is sent
through electrode lead 109 to an implanted electrode array 110.
Typically, the electrode array 110 includes multiple electrode
contacts 112 on its surface that provide selective electrical
stimulation of the cochlea 104.
[0006] FIG. 2 shows an example of an electrode contact
configuration used in a 12-channel electrode array as described in
U.S. Pat. No. 6,600,955. An electrode array containing 12 electrode
contacts 201 (black dots) is positioned within the scala tympani of
the cochlea. Each of these electrode contacts 201 is connected to a
capacitor C 203 and a pair of current sources 205 and 207, whereby
the second ports of current sources 205 and 207 are connected to
implant ground GND 209 and implant supply voltage V.sub.CC 211,
respectively. Current sources 205 and 207 may be implemented, for
example, using P-channel and N-channel MOS field effect
transistors, respectively. Thus, for convenience, the current
sources 205 and 207 are designated as P-sources and N-sources.
Reference electrode 213 is positioned outside the cochlea and
connected to a pair of switches 215 and 217, whereby the second
ports of switches 215 and 217 are connected to implant ground GND
and implant supply voltage V.sub.CC, respectively.
[0007] An audio signal, such as speech or music, can be processed
into multiple frequency band pass signals, each having a signal
envelope and fine time structure within the envelope. One common
speech coding strategy is the so called
"continuous-interleaved-sampling strategy" (CIS), as described by
Wilson B. S., Finley C. C., Lawson D. T., Wolford R. D., Eddington
D. K., Rabinowitz W. M., "Better speech recognition with cochlear
implants," Nature, vol. 352, 236-238 (July 1991), which is hereby
incorporated herein by reference. The CIS speech coding strategy
samples the signal envelopes at predetermined time intervals,
providing a remarkable level of speech understanding merely by
coding the signal envelope of the speech signal. This can be
explained, in part, by the fact that auditory neurons phase lock to
amplitude modulated electrical pulse trains (see, for example,
Middlebrooks, J. C., "Auditory Cortex Phase Locking to
Amplitude-Modulated Cochlear Implant Pulse Trains," J Neurophysiol,
100(1), p. 76-912008, 2008 July, which is hereby incorporated
herein by reference). However, for normal hearing subjects, both
signal cues, the envelope and the final time structure, are
important for localization and speech understanding in noise and
reverberant conditions (Zeng, Fan-Gang, et al. "Auditory perception
with slowly-varying amplitude and frequency modulations." Auditory
Signal Processing, Springer New York, 2005, 282-290; Drennan, Ward
R., et al. "Effects of temporal fine structure on the
lateralization of speech and on speech understanding in noise."
Journal of the Association for Research in Otolaryngology 8.3
(2007): 373-383; and Hopkins, Kathryn, and Brian Moore. "The
contribution of temporal fine structure information to the
intelligibility of speech in noise," The Journal of the Acoustical
Society of America 123.5 (2008): 3710-3710; and all of which are
hereby incorporated herein by reference in their entireties).
[0008] Older speech coding strategies mainly encode the slowly
varying band pass envelope information and do not transmit the fine
time structure of the band pass signal. Some more recent coding
strategies, for example, Fine Structure Processing (FSP), do also
transmit the fine time structure information. In FSP, the fine time
structure of low frequency channels is transmitted through Channel
Specific Sampling Sequences (CSSS) that start at negative to
positive zero crossings of the respective band pass filter output
(see U.S. Pat. No. 6,594,525, which is incorporated herein by
reference). The basic idea of FSP is to apply a stimulation
pattern, where a particular relationship to the center frequencies
of the filter channels is preserved, i.e., the center frequencies
are represented in the temporal waveforms of the stimulation
patterns, and are not fully removed, as is done in CIS. Each
stimulation channel is associated with a particular CSSS, which is
a sequence of ultra-high-rate biphasic pulses (typically 5-10
kpps). Each CSSS has a distinct length (number of pulses) and
distinct amplitude distribution. The length of a CSSS may be
derived, for example, from the center frequency of the associated
band pass filter. A CSSS associated with a lower filter channel is
longer than a CSSS associated with a higher filter channel. For
example, it may be one half of the period of the center frequency.
The amplitude distribution may be adjusted to patient specific
requirements.
[0009] FIGS. 3A-3B show two examples of CSSS for a 6-channel
system. In FIG. 3A, the CSSS's are derived by sampling one half of
a period of a sinusoid whose frequency is equal to the center
frequency of the band pass filter (center frequencies at 440 Hz,
696 Hz, 1103 Hz, 1745 Hz, 2762 Hz, and 4372 Hz). Sampling is
achieved by means of biphasic pulses at a rate of 10 kpps and a
phase duration of 25 .mu.s. For Channels 5 and 6, one half of a
period of the center frequencies is too short to give space for
more than one stimulation pulse, i.e., the "sequences" consist of
only one pulse, respectively. Other amplitude distributions may be
utilized. For example, in FIG. 3B, the sequences are derived by
sampling one quarter of a sinusoid with a frequency, which is half
the center frequency of the band pass filters. These CSSS's have
about the same durations as the CSSS's in FIG. 3A, respectively,
but the amplitude distribution is monotonically increasing. Such
monotonic distributions might be advantageous, because each pulse
of the sequence can theoretically stimulate neurons at sites which
cannot be reached by its predecessors.
[0010] FIG. 4 illustrates a typical signal processing
implementation of the FSP coding strategy. A Filter Bank 401
processes an audio input signal to generate band pass signals that
each represent a band pass channel defined by an associated band of
audio frequencies. The output of the Filter Bank 401 goes to a
Stimulation Signal Processor 400 that includes an Envelope Detector
402 that extracts band pass envelope signals reflecting time
varying amplitude of the band pass signals which includes
unresolved harmonics and are modulated with the difference tones of
the harmonics, mainly the fundamental frequency F0, and to a
Stimulation Timing Module 403 that generates stimulation timing
signals reflecting the temporal fine structure features of the band
pass signals. For FSP, the Stimulation Timing Module 403 detects
the negative to positive zero crossings of each band pass signal
and in response starts a CSSS as a stimulation timing signal. The
Stimulation Signal Processor 400 also includes a Pulse Generator
404 uses the band pass envelope signals and the stimulation timing
signals to produce the electrode stimulation signals for the
electrode contacts in the implant 405.
[0011] FSP and FS4 are the sole commercially available coding
strategies that code the temporal fine structure information.
Although they have be shown to perform significantly better than
e.g. CIS in many hearing situations, there are some other hearing
situations in which no significant benefit has been found so far
over CIS-like envelope-only coding strategies, in particular with
regard to localization and speech understanding in noisy and
reverberant conditions.
SUMMARY
[0012] Embodiments of the present invention are directed to systems
and methods for generating electrode stimulation signals for the
electrode contacts in a cochlear implant electrode array. A band
pass filter bank is configured for processing an audio input signal
to generate multiple band pass signals each representing an
associated band of audio frequencies in the audio input signal. A
stimulation signal processor is configured for generating electrode
stimulation signals for the electrode contacts based on the band
pass signals. For each electrode contact, the electrode stimulation
signal is a sequence of monophasic stimulation pulses varying in
polarity between positive polarity and negative polarity with
successive pulses separated in time by an interpulse interval
sufficient for neural response. A charge imbalance module is
configured for calculating accumulated charge imbalance and charge
imbalance polarity for each electrode contact after each
stimulation pulse. For each electrode contact, a stimulation pulse
has the same polarity as an immediately preceding stimulation pulse
for that electrode contact only when the charge imbalance polarity
has opposite polarity from the immediately preceding stimulation
pulse for that electrode contact, and the accumulated charge
imbalance exceeds a defined charge imbalance threshold value.
Otherwise, each stimulation pulse has the opposite polarity as the
immediately preceding stimulation pulse for that electrode
contact.
[0013] In further specific embodiments, the charge imbalance module
may be configured for calculating the accumulated charge imbalance
in terms of maximum comfortable level (MCL) for each electrode
contact and/or the defined charge imbalance threshold value may be
defined in terms of maximum comfortable level (MCL) for each
electrode contact. Each sequence of monophasic stimulation pulses
may end with a final charge balancing stimulation pulse having a
polarity and amplitude offsetting the accumulated charge imbalance
and charge imbalance polarity so that after the final charge
balancing stimulation pulse the accumulated charge imbalance is
zero.
[0014] The stimulation pulses may have a constant or variable pulse
width. And the interpulse interval may be a fixed time duration or
a variable time duration. The charge imbalance module may be
located in an implanted stimulation processor implanted under the
skin of a patient user, or in an external signal processor attached
to the skin of a patient user.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows anatomical structures of a human ear and some
components of a typical cochlear implant system.
[0016] FIG. 2 shows an example of an electrode contact
configuration used in a 12-channel electrode array
[0017] FIG. 3A shows channel specific sampling sequences (CSSS) for
two 6-channel systems utilizing biphasic pulses at 10 kpps and
phase duration of 25 .mu.s derived from a sinusoid within
[0-.pi.].
[0018] FIG. 3B shows channel specific sampling sequences (CSSS) for
two 6-channel systems utilizing biphasic pulses at 10 kpps and
phase duration of 25 .mu.s derived from a sinusoid within
[0-.pi./2], amplitudes monotonically increasing.
[0019] FIG. 4 shows various functional blocks in a signal
processing arrangement for a hearing implant.
[0020] FIG. 5 shows various logical steps in developing electrode
stimulation signals according to an embodiment of the present
invention.
[0021] FIG. 6 shows greater detail as to logical steps in
developing electrode stimulation signals according to an embodiment
of the present invention.
[0022] FIG. 7 shows various functional blocks in a signal
processing arrangement for a hearing implant according to an
embodiment of the present invention.
[0023] FIG. 8 shows an example of a short time period of an input
speech signal from a sensing microphone.
[0024] FIG. 9 shows the microphone signal decomposed by band-pass
filtering by a bank of filters.
[0025] FIG. 10 shows an example of a stimulation pulse sequence
according to an embodiment of the present invention.
DETAILED DESCRIPTION
[0026] Cochlear implants typically apply charge-balanced biphasic
or charge-balanced triphasic stimulation pulses for electrical
stimulation. It has been shown that anodic-first and cathodic-first
pulses result in different loudness percepts, probably related to
individual neural survival status. Also charge-balanced
pseudo-monophasic pulses (first phase high amplitude and short
duration, second phase low amplitude and long duration) have been
used in research where lower MCLs have been observed for
anodic-first pulse shapes. Lowest MCL thresholds have been found
for alternating monophasic waveforms where two succeeding
monophasic pulses of alternating polarity and same absolute
amplitude were applied with 5 ms inter-pulse gap (sufficient for
neural response). But charge balancing is required for safety
reasons so pure monophasic stimulation with independent amplitudes
has not been considered usable in humans. But embodiments of the
present invention introduce a novel and inventive form of
monophasic stimulation that is charge balanced over time. This is
also the most efficient waveform for stimulation so considerably
lower energy is used for stimulation without restrictions in
perception.
[0027] FIGS. 5 and 6 are flow charts showing various logical steps
and FIG. 7 shows various functional blocks in a signal processing
arrangement for a cochlear implant that produces electrode
stimulation signals to electrode contacts in an implanted cochlear
implant array according to an embodiment of the present invention.
A pseudo code example of such a method can be set forth as:
Band Pass Processing:
[0028] BandPassFilter (input_sound, bp_signals)
Accumulated Charge Imbalance:
[0029] ChargeImbalance (stim_signals, accum_charge_imbalance,
charge_imbalance_polarity)
Pulse Generation:
[0030] PulseGenerate (bp_signals, accum_charge_imbalance,
charge_imbalance_polarity, stim_signals) The details of such an
arrangement are set forth in the following discussion.
[0031] In the arrangement shown in FIG. 7, the audio input signal
is produced by one or more sensing microphones, which may be
omnidirectional and/or directional. Filter Bank 701 processes the
audio input signal, step 501, with a bank of multiple parallel band
pass filters, each of which is associated with a specific band of
audio frequencies; for example, using a filter bank with 12 digital
Butterworth band pass filters of 6th order, Infinite Impulse
Response (IIR) type, so that the audio input signal is filtered
into some K band pass signals, U.sub.1 to U.sub.K where each signal
corresponds to the band of frequencies for one of the band pass
filters. Each output of the Filter Bank 701 can roughly be regarded
as a sinusoid at the center frequency of the band pass filter which
is modulated by an amplitude envelope signal. This is due to the
quality factor (Q.apprxeq.3) of the filters. In case of a voiced
speech segment, the band pass envelope is approximately periodic,
and the repetition rate is equal to the pitch frequency.
Alternatively and without limitation, the Filter Bank 701 may be
implemented based on use of a fast Fourier transform (FFT) or a
short-time Fourier transform (STFT). Based on the tonotopic
organization of the cochlea, each electrode contact in the scala
tympani typically is associated with a specific band pass channel
of the Filter Bank 701. The Band Pass Filter Bank 701 also may
perform other initial signal processing functions such as automatic
gain control (AGC) and/or noise reduction.
[0032] FIG. 8 shows an example of a short time period of an audio
input signal from a sensing microphone, and FIG. 9 shows the
microphone signal decomposed by band-pass filtering by a bank of
filters. An example of pseudocode for an infinite impulse response
(IIR) filter bank based on a direct form II transposed structure is
given by Fontaine et al., Brian Hears: Online Auditory Processing
Using Vectorization Over Channels, Frontiers in Neuroinformatics,
2011; incorporated herein by reference in its entirety:
TABLE-US-00001 for j = 0 to number of channels - 1 do for s = 0 to
number of samples - 1 do Y.sub.j(s) = B.sub.0j *X.sub.j(s) +
Z.sub.0j for i = 0 to order- 3 do Z.sub.ij = B.sub.i+l, j
*X.sub.j(s) + Z.sub.i+l,j - A.sub.i+l, j * Y.sub.j(s) end for
Z.sub.order-2,J = B.sub.order- 1,,j * X.sub.j(s) -A.sub.order-1,j *
Y.sub.j(s) end for end for
[0033] The band pass signals U.sub.1 to U.sub.K (which can also be
thought of as electrode channels) are output to a Stimulation
Signal Processor 700 that generates the electrode stimulation
signals for each electrode contact, step 502. Specifically, the
Stimulation Signal Processor 700 includes an Envelope Detector 702
which extracts characteristic band pass envelope signals outputs
Y.sub.1, . . . , Y.sub.K that represent the channel-specific time
varying amplitudes of the band pass signals U.sub.1 to U.sub.K. The
envelope extraction can be represented by Y.sub.k=LP(|U.sub.k|),
where |.| denotes the absolute value and LP(.) is a low-pass
filter; for example, using 12 rectifiers and 12 digital Butterworth
low pass filters of 2nd order, IIR-type. A properly selected
low-pass filter can advantageously smooth the extracted envelope to
remove undesirable fluctuations. Alternatively, if the band pass
signals U.sub.1, . . . , U.sub.K are generated by orthogonal
filters, the Envelope Detector 702 may extract the Hilbert
envelope. In some embodiments, the Envelope Detector 702 may also
be configured to determine one or more other useful features of the
band pass envelope such as envelope slope (e.g., based on the first
derivative over time of the envelope), envelope peak (ascending
slope/positive first derivative followed by descending
slope/negative first derivative), and/or envelope amplitude of the
band pass envelope.
[0034] A Stimulation Timing Module 703 in the Stimulation Signal
Processor 700 processes the band pass signals on a regular time
grid (e.g. 1000 pps) based on selected temporal fine structure
features such as negative-to-positive zero crossings to generate
band pass timing pulses. In some embodiments, the Stimulation
Timing Module 703 may limit the instantaneous band pass frequency
f.sub.0 to the upper and lower frequency boundaries f.sub.L1 and
f.sub.U1 of the respective filter band. For example, a given band
pass signal may have a lower frequency boundary f.sub.L1 of 700 Hz
and an upper frequency boundaries of f.sub.U1=770 Hz.
[0035] The Stimulation Signal Processor 700 also includes a Pulse
Generation Module 704 that generates the electrode stimulation
signals for the electrode contacts in the Implant 705 by generating
one or more corresponding stimulation pulses for each band pass
signal. For each electrode contact, the electrode stimulation
signal is a sequence of monophasic stimulation pulses that vary in
polarity between positive polarity and negative polarity with
successive pulses separated in time by an interpulse interval
sufficient for neural response.
[0036] A Charge Imbalance Module 706 is configured for calculating
accumulated charge imbalance and charge imbalance polarity, step
503, which are used by the Pulse Generation Module 704 to generate
the electrode stimulation signals. In specific embodiments, the
Charge Imbalance Module 706 may be located in an implanted
stimulation processor implanted under the skin of a patient user,
or in an external signal processor attached to the skin of a
patient user. And the Charge Imbalance Module 706 may be
specifically configured for calculating the accumulated charge
imbalance in terms of maximum comfortable level (MCL) for each
electrode contact and/or the defined charge imbalance threshold
value may be defined in terms of maximum comfortable level (MCL)
for each electrode contact.
[0037] The details of generating the electrode stimulation signals
are shown by the flowchart blocks in FIG. 6 where the first two
steps specifically are, for each electrode contact after each
stimulation pulse, the Charge Imbalance Module 706 calculates the
accumulated charge imbalance, step 601 and the charge imbalance
polarity, step 602. Then for each electrode contact, the Pulse
Generation Module 704 generates the stimulation pulse sequence
using a two-step decision process. The Pulse Generation Module 704
generates the next stimulation pulse with the same polarity as an
immediately preceding stimulation pulse for that electrode contact
only when the charge imbalance polarity has opposite polarity from
the immediately preceding stimulation pulse for that electrode
contact, step 603, and the accumulated charge imbalance exceeds the
defined charge imbalance threshold value, step 605. Otherwise, each
stimulation pulse has the opposite polarity as the immediately
preceding stimulation pulse for that electrode contact. So if the
charge imbalance polarity is not opposite the polarity of the
preceding stimulation pulse in step 603, then the Pulse Generation
Module 704 generates the next stimulation pulse in the sequence
with opposite polarity from that of the preceding stimulation
pulse, step 604. Or if the charge imbalance polarity is opposite
from the polarity of the preceding stimulation pulse in step 603,
but the accumulated charge imbalance is less than some a defined
charge imbalance threshold value (e.g., 50% of MCL) in step 605,
then the Pulse Generation Module 704 still generates the next
stimulation pulse in the sequence with opposite polarity from that
of the preceding stimulation pulse in step 604.
[0038] FIG. 10 shows an example of a stimulation pulse sequence on
a single electrode contact according to an embodiment of the
present invention. In this example, the defined charge imbalance
threshold value is set to 50% MCL. Positive polarity stimulation
pulses contribute to the accumulated charge imbalance with positive
sign, while negative polarity stimulation pulses contribute with
negative sign. In the example in FIG. 10, the stimulation pulse
amplitudes also are given in terms of percentage relative to MCL.
FIG. 10 shows amount of accumulated charge imbalance below each
stimulation pulse in terms of percentage of MCL. The stimulation
pulses may have a constant or variable pulse width, and the
interpulse interval may be a fixed time duration or a variable time
duration depending on the specific selected signal coding
strategy.
[0039] The first stimulation pulse in FIG. 10 is applied with
positive polarity and an amplitude of 100% MCL, followed by a
stimulation pulse with opposite polarity at 70% MCL. The
accumulated charge imbalance afterwards is 30% MCL (100-70). After
applying two more stimulation pulses, the charge imbalance polarity
is opposite from the polarity of the preceding stimulation pulse
(the fourth pulse in the sequence) and the accumulated charge
imbalance also has increased to 60% MCL (100-70+90-60), thus
exceeding the 50% MCL that is the defined charge imbalance
threshold value. Therefore the polarity of the fifth stimulation
pulse does not switch, but remains the same negative polarity as
the immediately preceding stimulation pulse to reduce the
accumulate charge imbalance to -30% MCL. The sequence of
stimulation pulses then resume with alternating polarity until the
charge imbalance polarity again is opposite from the polarity of
the preceding stimulation pulse and the accumulated charge
imbalance also again exceeds the defined charge imbalance threshold
value after the twelfth stimulation pulse (-70% MCL). Then the
thirteenth stimulation pulse will remain at the same positive
polarity as the preceding twelfth stimulation pulse, again reducing
the accumulated charge imbalance (to zero). The same thing happens
after the seventeenth stimulation pulse with the following
eighteenth stimulation pulse (-60% MCL).
[0040] In the example shown in FIG. 10, the time sequence of
stimulation pulses ends with a final charge balancing stimulation
pulse having a polarity and amplitude that offsets the accumulated
charge imbalance and charge imbalance polarity so that after the
final charge balancing stimulation pulse the accumulated charge
imbalance is zero.
[0041] The Pulse Generation Module 704 also will typically further
adjust output the electrode stimulation signals based on a
non-linear mapping that reflects patient-specific scaling from the
fitting process, e.g., THR and MCL. Instead of applying a single
output stimulation pulse for each selected timing pulse, the Pulse
Generation Module 704 can use frequency specific pulse sequences
for one or more selected electrode contacts. Such pulse sequences
can vary in inter-pulse intervals and amplitude shape. Amplitude
shapes can be based on templates, or the amplitudes can fall with a
decay, e.g. with an exponential characteristic. In some
embodiments, rather than generating a single output stimulation
pulse for each selected timing pulse, the Pulse Generation Module
704 may excite an output pulse oscillator with the selected timing
pulses. For example, such output pulse oscillators can be damped
oscillators with electrode specific resonance frequencies; for
example, the center frequencies assigned to each electrode contact.
The oscillation then provides amplitudes for stimulation pulses
which are applied on pulse sequences.
[0042] In some embodiments, the Pulse Generation Module 704 can be
configured to apply the electrode stimulation signals via virtual
channels (two simultaneous neighboring channels). So if first
electrode contact E1 is assigned to a frequency band of 100 to 200
Hz and the second E2 to 200 to 300 Hz, then an instantaneous
frequency of, for example, 200 Hz would lead to stimulation
AMP1=(MCL1-THR1)/2+THR1 and AMP2=(MCL2-THR2)/2+THR2. This would
allow a fine spectral and temporal representation of the output
stimulation pulses.
[0043] Embodiments of the invention may be implemented in part in
any conventional computer programming language such as VHDL,
SystemC, Verilog, ASM, etc. Alternative embodiments of the
invention may be implemented as pre-programmed hardware elements,
other related components, or as a combination of hardware and
software components.
[0044] Embodiments can be implemented in part as a computer program
product for use with a computer system. Such implementation may
include a series of computer instructions fixed either on a
tangible medium, such as a computer readable medium (e.g., a
diskette, CD-ROM, ROM, or fixed disk) or transmittable to a
computer system, via a modem or other interface device, such as a
communications adapter connected to a network over a medium. The
medium may be either a tangible medium (e.g., optical or analog
communications lines) or a medium implemented with wireless
techniques (e.g., microwave, infrared or other transmission
techniques). The series of computer instructions embodies all or
part of the functionality previously described herein with respect
to the system. Those skilled in the art should appreciate that such
computer instructions can be written in a number of programming
languages for use with many computer architectures or operating
systems. Furthermore, such instructions may be stored in any memory
device, such as semiconductor, magnetic, optical or other memory
devices, and may be transmitted using any communications
technology, such as optical, infrared, microwave, or other
transmission technologies. It is expected that such a computer
program product may be distributed as a removable medium with
accompanying printed or electronic documentation (e.g., shrink
wrapped software), preloaded with a computer system (e.g., on
system ROM or fixed disk), or distributed from a server or
electronic bulletin board over the network (e.g., the Internet or
World Wide Web). Of course, some embodiments of the invention may
be implemented as a combination of both software (e.g., a computer
program product) and hardware. Still other embodiments of the
invention are implemented as entirely hardware, or entirely
software (e.g., a computer program product).
[0045] Although various exemplary embodiments of the invention have
been disclosed, it should be apparent to those skilled in the art
that various changes and modifications can be made which will
achieve some of the advantages of the invention without departing
from the true scope of the invention.
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